Billion 7800dxl Plex-battlefield 4

[#1]

Hi i am a noob when it comes to networking  :D. I have connected the bt fiber router to my billion 7800dxl as i was having issues with stock router provided (not connecting to chromecast etc). I am currently trying to port forward both my plex server and battlefield 4 with no success can anyone plz help. Is the problem going through two routers (Double-NAT) or should it just work?

[#2]

Please remember im a noob no big words plz 

[#3]

Is it possible for me to use my billion 7800dxl and port forward on fibre.